The present invention concerns a method for producing selectively controlled pressure pulses in a mass of gas, in particular contained in a space of large dimensions. By a mass of gas is here also included a mixture of gases e.g. air.
The invention also concerns a device for performing said method.
By generating pressure pulses in a mass of gas it undergoes a treatment raising the mechanical energy of the gas. The increased energy appears as potential energy in the form of local pressure fluctuations and as kinetic energy due to local speed variations in the mass of gas. The energy in the pulses oscillates between zero and a maximum, the two forms of energy being in phase with each other.
The energy of the pressure pulses can under certain conditions be used for different purposes such as preventing particles in the gas from settling on the walls of the space in which it is contained, as well as removing such particles already settled on said walls as a coating. The pulses can also be used for promoting the mixing of two different gaseous media, for mixing a gas with fluid droplets or solid particles and for other aspects of homogenizing a gas. The utilization of pressure pulses thus can be applied for cleaning purposes and in different stages in e.g. the process industry for treating gases that are going to be mixed, be combusted, react chemically, perform work etc. as well as treating media in the form of solid particles or fluid droplets suspended in a gas.
A condition for making such treatments of a mass of gas possible is that the pulses have a considerable acoustic power.
For the treatment of air in such applications as mentioned above the best results are achieved when the pulses are of a frequency near the lower limit of audible sound. At these low frequencies the pulses are not damped out to the same extent as at higher frequencies. Furthermore the long wave length enables the pulses to propagate around obstructing partitions reaching all the parts of the space concerned at uniform level of acoustic pressure.
Known methods for generating pressure pulses or sound waves in order to treat a mass of gas have not been able to produce pulses of enough energy for satisfactory utilizations of the kinds mentioned above when the gas is contained in a space of large dimensions.
The use of pressure pulses generated in a mass of gas has for example been applied for cleaning the walls of constructions containing gas. The absence of a pulse generator of sufficient efficiency, however, has limited the application to the cleaning of relatively small such constructions.
Examples of cleaning by use of sound pulses are disclosed in the Swedish patent application document 80 07 150-9 (with the publication No. 425 597) and in the British patent specification No. 2 033 130.
With the methods disclosed in said references, pulses having relatively high, but for many purposes still too low, energy are produced.
In the method disclosed in the Swedish patent document 80 07 150-9 the pressure difference between two spaces, periodically brought in communication with each other, is used, whereby the pulses are generated, as gas due to the pressure difference flows from one space to the other. The pulse generator includes a pipe for pressurized gas provided with a rotating cylindrical valve driven by an engine. The pipe and the valve, which are coaxially arranged, are each provided with a slot. As the slot of the valve during the rotation passes the slot of the pipe, communication is established between the pipe and the surrounding, whereby gas flows out through the aligned slots, generating a pulse. The pulses are then amplified in a resonance tube. The frequency is about 20 Hz. With this device a wave of substantially sinusodial shape is received, which results in an unfavourable distribution of energy during the pulse period. To maintain the pressure difference between the inside of the pipe and the surrounding a continuous supply of pressurized gas is necessary. The compressor required for the production of pressurized gas thus has to work all the time against the pressure in the gas pipe. This therefore requires a relatively high effective power input for the compression work in relation to the received acoustic power. A great part of this work is lost as heat. Furthermore, pulses are generated when compressing the gas. Due to the valve, these pulses do not leave the pipe, so that their energy is not made use of. Also this energy is lost as heat. The low acoustic efficiency of this method economically and practically limits the achievable output power.
Also with the sound generating device of the British patent specification 2 033 130 the sound pulses are generated by the flow of gas through an opening between two spaces of different pressure periodically brought in communication with each other. In this case the opening is controlled by a reciprocating slide connected to a membrane at the closed end of a resonance tube. In a starting position when the slide limits the opening to a narrow slot a soft low frequency sound is generated, affecting the membrane to oscillate at a frequency determined by the resonance tube. This forces the slide to reciprocate at the same frequency, closing and opening the valve opening, whereby the sound pressure in the resonance tube increases. This sound generator suffers from the same described drawbacks as the device of the Swedish patent document 80 07 150-9 does. An advantage, however, is received by the positive feed-back through the membrane securing harmony between the resonance frequency of the tube and the pulse frequency.
By the method disclosed in the referred British patent specification 2 033 130 an acoustic power of 1.5 kW at the highest is reached. It is intended to be used, as an alternative to soot blowing, for cleaning equipment in steam boiler plants such as superheater, heat exchanger, economizer and preheater. With the limited power output of the pulse generator this method can be used only for relatively small plants. For the cleaning of economizer and superheater in a boiler of more than 300 MW the power of the generated pulses is insufficient.